1. Field of the Invention
The present invention relates to satellite to satellite communication systems. More specifically, the present invention relates to acquisition and tracking systems for satellite to satellite communication systems.
2. Description of the Related Art
In certain applications, satellite to satellite communication is necessary in order to complete a communication link to a ground station. Communication satellite cross-links are currently provided by radio-frequency (RF) transceivers.
While RF technology is technically mature, these systems suffer from certain shortcomings. One shortcoming results from the fact that some RF systems are generally simplex systems allowing communication in one direction at a time. Duplex systems are generally much more costly than simplex systems and are therefore currently of limited application.
In addition, the frequencies available for RF satellite communication is currently very limited. To the extent that a frequency band is available, governmental licenses may be required.
Finally, RF systems offer limited rates for data transmission.
High data rate station to station optical communication through free space is afforded by the use of a very narrow optical beam. Acquisition and tracking of the narrow beam is problematic in that the beam must be pointed at a remote transceiver with microradian accuracy. Optical beams of sufficient brightness are typically tens of microradians in diameter, while the corresponding requirement for RF beamwidths is generally on the order of one to two degrees.
Prior optical communications concepts embody power consuming optical beacons for initial acquisition. An alternative approach involves the scanning of a diffraction limited transmit beam over the region of pointing uncertainty in order to illuminate the remote transceiver. Since the narrow angle transmit beam of both transceivers must be scanned and finally co-aligned (while taking into account the point-ahead angle), the acquisition process was time consuming, to the detriment of the revenue-bearing data stream.
In addition, prior concepts embodied the idea of each transceiver autonomously tracking the image of the other's transmit beam. Many proposed optical communications systems incorporate an expensive charge coupled device (CCD) image sensor or an image-splitting device followed by a quadrant detector. (See "Lightweight Lasercomm Terminal Concept for LEO Orbit Satellite Constellations"; Marshalek, Begley; Ball Aerospace Systems Group-Paper 2123-18, OE/LASE '94, Jan. 22-29, 1994 and "Laser Transmitter Aims at Laser Beacon", Hemmati, Lesh; NASA Tech Briefs, November, 1993.) Since the dynamic range of the pointing error is large (e.g., 1,000:1), a single-stage tracker is not feasible, especially when the communication data rates are considered. Therefore, a coarse-fine implementation with dual detector complements has been the usual approach. However, these systems are typically complicated and expensive.
Thus, there has been a need in the art for an inexpensive system for effecting acquisition and tracking of an optical beam.